In many industries, materials are stored in holding areas, either as waste or for future use. Due to the hazardous nature of certain materials, geomembrane liners, steel liners, or concrete liners may be installed along the boundaries of holding areas to create an impermeable barrier between the materials and the surrounding environment. For example, liners have been installed along the boundaries of landfills, surface impoundments, water reservoirs, tanks, and holding ponds to prevent the contained material from leaking into the soil and groundwater. Where the contained material is liquid or semi-liquid, the majority of facilities employ geomembrane liners, which are typically fabricated from large sheets of flexible material, such as plastic, and may be comprised of two or more layers.
Many systems have been developed to monitor the integrity of geomembrane liners and to discover and locate leaks by way of electrical detection systems. These systems generally require an electrically conductive medium either covering the liner or imbedded within the liner system. For example, U.S. Pat. No. 5,661,406 to Daily, et al. discloses a method for detecting and locating leaks in a liner using electrical potential and electrical resistance measurements in the subsurface around the periphery of the liner, under the liner, or within the material holding area. In each case, the contained material must be electrically conductive. In a first embodiment, Daily teaches exciting the contained material, which is electrically isolated from the sub-surface outside the containment area by an insulating geomembrane liner, to an electrical potential above the surrounding soil, and sensing any current flow through a leak in the liner by measuring the electrical potential at a series of electrodes placed within the contained material and/or in the surrounding soil. In a second embodiment, Daily teaches the use of electrical resistivity tomography (ERT) to detect leak points in a liner by constructing a base-line resistivity image for the surrounding sub-surface and comparing subsequent resistivity images. The electrical resistivity profile identifies changes in the subsurface electrical characteristics caused by the flow of a conductive liquid through the liner.
U.S. Pat. No. 5,288,168 to Spencer is an example of constructing the leak detection system within the geomembrane liner itself. Spencer discloses a thermoplastic liner having upper and lower plastic layers. The lower plastic layer is adapted to sufficiently conduct electricity to enable the detection of pin hole leaks in the liner for in situ electrical analysis, and the upper layer provides strength and support. Pin hole leaks are detected by moving a spark discharge probe along the upper surface of the liner and detecting a spark discharge between the probe and the conductive lower layer. In this way, the contained material is not required to be electrically conductive.
U.S. Pat. No. 4,947,470 to Darilek discloses the use of a two-layered liner with an inter-liner zone between the layers. The top and bottom layers are made from electrically resistive material, such as polyethylene, while the inter-liner zone contains a conductive material, such as water or moist sand. An electrode is placed within the impounded material, and a number of detectors are disposed in an array on one side of the liner. A voltage is impressed across the liner, and the detector array monitors for the presence of an electromagnetic field created by a current flowing through a leak in the liner. The leak is located with orthogonal measurements from selected detectors, and these measurements are used to geometrically locate the leak. U.S. Pat. No. 5,362,182 to Hergenrother teaches a similar multi-layer liner having an electrically conductive layer sandwiched between two electrically non-conductive insulating layers. Pairs of electrical contacts are embedded within the electrically conductive monitoring layer, and a resistivity meter is connected between any pair of contacts to measure resistivity changes in the layer caused by water seepage. Alternatively, a voltage unit is used to drive current through the conductive layer and changes in the current within the electrically conductive layer are monitored.
Time domain reflectometry (TDR), an electric analog to a radar system, is an electrical pulse testing technique historically developed to locate breaks in transmission cables by measuring the arrival time of reflected electrical energy. Coaxial TDR cables are usually constructed with a central metallic conductor surrounded by an electrically non-conductive insulating material (the dielectric), a metallic outer conductor surrounding the insulation, and a protective jacket. The cables have characteristic impedance determined by the thickness and type of insulating material between the cables. A change in the distance between the inner and outer conductors (e.g., a crimp or break in the cable) or a foreign substance in the insulating material (e.g., moisture) will change the impedance of the cable. Electromagnetic pulses generated along the cable are reflected by the greater impedance, and the time delay and wave shape of the reflected electrical energy is determinative of the location and severity of damage to the cable. An alternate embodiment of the TDR cable system is a transmission line comprised of a pair of parallel, spaced apart, electrically conductive wires, which are separated and surrounded by an insulating medium, and enclosed in a protective jacket.
"Cable-radar" TDR systems are used in many applications for detecting moisture, including sensing the level of fluid in a tank, soil conditions, leaking pipes, and discontinuities in pond liners. For example U.S. Pat. No. 5,648,724 to Yankielun, et al., discloses embedding a TDR transmission line in a serpentine pattern within a roof to detect the location of leaks. It is also known to dispose TDR transmission lines under a liner to detect leaks. Unfortunately, where the leaking fluid does not intersect the path of the transmission line, the leak will not be detected.
The present invention is a method and apparatus for detecting and locating leaks in a geomembrane liner that overcomes the problems associated with TDR transmission line systems by expanding the TDR cable technology to a two-dimensional TDR system that is uniquely applied to a specially designed geomembrane liner.
Therefore, in view of the above, a basic object of the present invention is to provide a geomembrane liner system designed to employ two-dimensional time domain reflectometry for in-situ monitoring of the integrity of the liner.
Another object of the present invention is to provide a reliable and efficient method and apparatus for detecting and locating leaks within a geomembrane liner using TDR.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.